The present invention relates to a mobile satellite communication terminal equipment and, particularly, to a mobile satellite communication terminal equipment which can also be used as a ground mobile terminal performing communication through only ground facilities.
It is well known that, as a communication system to be used in a mobile telephone or portable telephone, there are two kinds of system, a ground mobile communication system (cellular system) which utilizes only ground facilities and a mobile satellite communication system which utilizes a satellite.
In the ground mobile communication system such as mobile telephone system which has been popularized recently, a mobile telephone which is the mobile communication terminal communicates, through radio wave, together with a nearest radio base station which is connected to a fixed telephone network through an upper exchange station, so that the mobile telephone can communicate with a subscriber of the network.
On the other hand, although the mobile satellite communication system is inoperable in an area such as underground area, interior of a building or area under an overhead construction which is out of line-of-sight of a satellite, it is operable in any out-of-way area such as solitary island or mountain area in which the ground communication system can not provide services economically, so long as the out-of-way area is on a direct line-of-sight of the satellite. Therefore, the mobile satellite communication system having a wide service area is expected as a complementary system for the ground mobile communication system.
Particularly, in a mobile satellite communication system utilizing a Low Earth Orbit (LEO) satellite in which a distance of a mobile ground terminal measured from the LEO satellite is short and a beam irradiation range is narrow compared with a stationary satellite, a communication is possible with lower receiving sensitivity and smaller transmission power of the terminal. Therefore, it is possible to reduce a size and weight of the mobile terminal and power consumption thereof. Thus, the mobile terminal of the satellite system can be realized with the size and cost compatible with those of the mobile terminal of the ground communication system.
However, the mobile ground communication system and the mobile satellite communication system had been developed separately from each other and an antenna, a high frequency portion and a radio control portion of the mobile terminal of each system are assembled as a discrete integral unit.
Therefore, when a user moves from an area in which either one of the mobile ground communication and the mobile satellite communication is possible to another area in which only the other communication is possible, he must bring the mobile terminals of the both systems with him. This is troublesome for the user.
In order to solve this problem, Japanese Patent Application Laid-open No. Hl-248715 (1989) proposes a single mobile terminal which can be connected to either of the ground system and the satellite system. That is, as shown in FIG. 1, the proposed mobile terminal comprises a satellite system antenna 101, a ground system antenna 102, a satellite system high frequency portion 103 connected to the satellite system antenna 101, a ground system high frequency portion 104 connected to the ground system antenna 102, a receiving IF switch circuit 105 for selecting an output intermediate frequency signal of either one of the high frequency portions 103 and 104, a transmitting IF switch circuit 106 for selectively outputting a transmitting intermediate frequency signal to either one of the high frequency portions 103 and 104, a demodulator 107 for demodulating the intermediate frequency signal from the receiving IF switch circuit, a modulator 108 for modulating a transmitting signal, a baseband processor 109 for processing signals according to a protocol of a selected one of the satellite system and the ground system, a telephone set 110 and a portable computer 111.
According this mobile terminal, the baseband processor, the telephone set and the portable computer which are related to a signal processing subsequent to the IF signal are used commonly for the satellite system and the ground system.
However, since the antennas and the high frequency portions of the respective systems are incorporated, both the transmitting and receiving systems must be transported even when only one of the ground and satellite systems is used. Further, since both of the receiving IF switch circuit and the transmitting IF switch circuit are necessary, the terminal can not be enough compact and enough light-weight, causing the portability thereof to be low and the reduction of cost to be impossible.
Further, the mobile satellite communication system can not be used in an area such as underground area, interior of a building or area under an overhead construction which is out of line-of-sight of a satellite and, since a communication from an office in a country or area in which the ground communication system can not work effectively is impossible unless the satellite is on a line-of-sight of the terminal, the terminal must be moved close to a window of the office.